This invention relates generally to the photographic field and, more particularly, an improved method and exposure control system for automatically and selectively controlling the filtering of light incident upon a photoresponsive element during exposure.
Automatic light responsive control systems are well known in the photographic arts. Essentially, these systems evaluate scene brightness levels of illumination for controlling exposure parameters, including effective aperture size and exposure interval, as a function of scene brightness evaluated against the sensitometric characteristics of the film being used. Typically, scene brightness evaluation is performed with light-measuring circuits including one or more photoresponsive elements.
One such automatic exposure control system employs scanning type shutter blades. Exemplary scanning shutter blades usable in exposure control systems are generally disclosed in U.S. Pat. No. 3,942,183, issued Mar. 2, 1976, to George Whiteside; and U.S. Pat. No. 4,104,653, issued Aug. 1, 1978, to Bruce K. Johnson et al., all of which are presently assigned with the present application. As described in these patents, there are cooperating pairs of primary and secondary apertures formed in the shutter blades. These pairs of apertures cooperate respectively for blocking and unblocking the passage of light through an exposure opening to a film plane and through a photocell opening to a light sensing or photoresponsive cell used for controlling blade positioning. During the exposure cycle, the secondary apertures operate in conjunction with the photocell and a control circuit to define both the aperture values achieved and the exposure interval as a function of the amount of light received through the secondary apertures. In such systems, photoresponsive elements of the silicon type are commonly used because they, among other things, have excellent long term stability and linearity of output signal with input light power changes.
For optimizing the quality of the resultant photographs when using automatic exposure control systems, such as the type noted, it is known to employ spectral correction filters to correlate the spectral sensitivity curve of the photoresponsive element more closely with that of the photographic color film. Without such a filter, the photocell would react to the light frequencies, such as infrared (IR), and cause the control circuit to terminate exposure earlier than desired. This is especially the case when the photocell is of the silicon type, because such a photocell tends to be red (IR) sensitive. For providing the desired correction, a spectral correction filter is interposed in the photocell's optical path, for example, as described in U.S. Pat. No. 3,903,413, issued on Sept. 2, 1975, to Monis Manning; and commonly assigned with the present application. This patent discloses use of a silicon photodiode, sensitive to radiant energy between about 350 nm and 1200 nm, whereas the sensitivity of typical color photographic film is confined to the visible region of the spectrum, i.e., from about 400 nm to about 700 nm. Use is made of a spectral correction filter with peak absorption in the near-infrared region (700-1200 nm) and high transmission in the visual region to correct or generally match the spectral response of the photocell in relation to the film. This matching of sensitivities is particularly useful in cameras employing diffusion transfer photographic processes of the so-called "instant photography" type where errors in the exposure cannot be later compensated for as is possible with film subsequently developed in a photographic laboratory.
While use of infrared filters serve satisfactorily, complications can arise when reflectivities of different objects in photographic scenes exhibit widely disparate values, for example, where the exposure of the subject's facial skin is adversely affected because of the widely disparate reflectivities of the surrounding clothing or other objects, particularly in close-up situations. Partly as a result of this, it has been found advantageous to remove the infrared filter in flash exposure modes of operation.
Consequently, although retention of spectral correction filter in the photocell's optical path has been practiced during both ambient and artificial illumination modes, as disclosed in U.S. Pat. No. 4,040,070, issued on Aug. 2, 1977, to W. Hochreiter et al., a spectral correction filter is removed from the photocell's optical path when the flash mode is desired. This removal is achieved, upon attaching a flash unit to the camera, by actuation of a relatively complicated mechanical arrangement which swings the filter out of the optical path.
Still other known prior art is described in U.S. Pat. No. 3,468,228, issued on Sept. 23, 1969, to Howard G. Rogers, which provides automatic sequential positioning of a pair of dual filters over photocell and exposure apertures to attain a color balanced exposure of photosensitive material.